Project Summary/Abstract Temporal lobe epilepsy (TLE) is the most common form of focal epilepsy characterized by unprovoked, spontaneous seizures originating in the temporal lobe of the brain. 30% of patients do not gain seizure control through medication, leading to the remaining treatment option of surgical resection of the epileptic tissue. Improved therapies are sorely needed, including during epileptogenesis, the process of developing spontaneous seizures characteristic of chronic epilepsy. In resected tissue from TLE patients, deficits in neuronal inhibition have been observed at multiple levels, including cellular death in interneuron populations, alterations in levels of ?-aminobutyric acid receptors (GABAARs), and lesser studied reductions in chloride co- transporter function. In the majority of mature neurons within the brain, the electroneutral K+/Cl- co-transporter 2 (KCC2) promotes hyperpolarizing GABAAR responses through coupling the outward K+ gradient to Cl? efflux. When KCC2 transport is reduced, chloride loading of neurons results in depolarizing or even excitatory GABAAR responses. Reductions in KCC2 levels and depolarizing GABAAR responses have been observed in both resected tissue from human TLE patients and from animal models of TLE. However, the role of KCC2 deficits in epileptogenesis remains poorly understood. We will test the hypothesis that ?Decreased KCC2 function results in the development of pharmacoresistant seizures and anatomical changes characteristic of temporal lobe epilepsy.? I will use a novel flox-KCC2 mouse developed by our group to delete KCC2 transport function in specific neuronal populations within the brain in vivo. Experiments using both in vivo and in vitro electrophysiology will demonstrate how deficits in KCC2 transport affect network/neuronal excitability and the development of seizure refractoriness to anti-epileptic drugs. Preliminary experiments demonstrate that deletion of KCC2 in the hippocampus of adult mice results in spontaneous seizures. Further experiments will explore how this reduction in KCC2 transport affects markers of epileptogenesis. Collectively these experiments will provide new mechanistic insights into how deficits in KCC2 transport directly contribute to the pathophysiology of TLE and may aid the development of more efficacious treatments to limit the development of TLE and other epilepsies.